A photolithographic tool is a machine for applying a pattern onto a substrate, in particular a desired portion thereof. Such machines can be used, for example, in the fabrication of integrated circuits (ICs). In such cases, it could be considered that a pattern on a mask or reticle is used to form a pattern for a single IC layer. This pattern can be imaged onto a target site (e.g. including one or more pins) of a substrate (e.g., a silicon substrate). In particular, in the field of light-emitting diode (LED) manufacturing, a pattern of dense holes is first imaged onto a patterned sapphire substrate.
Compared with stripes, the imaging of dense holes or circular pillars is much more difficult and tends to suffer from a much reduced depth of focus (DoF). Therefore, the photolithographic process is imposed with very high requirements on focal plane control. On the other hand, in contrast to silicon wafers, patterned sapphire substrates are hard in nature and tend to have significant warpage, which contribute to further reductions in the useful DoF (UDoF). With an insufficient UDoF, defocus arising from errors in the focal plane control by the photolithographic system may lead to a degree of inhomogeneity of the pattern image on the substrate that is beyond a tolerable range, which is manifested as a macro effect that portions of the pattern image on the substrate have varying degree of brightness even under the same lighting conditions, i.e., a phenomenon that “inconsistency” or “color differences” can be perceived with the naked eye. As the naked eye is very sensitive to such “inconsistency” or “color differences”, visual inspection serves as an important check in the production line. Therefore, the control of various process parameters, especially the focal plane, is posed with highly demanding requirements.
Referring to FIG. 1, a conventional illumination system employs a light-homogenizing design based on a quadrilateral light-homogenizing rod 10. FIG. 2 shows conventional quadrilateral masks 20 which can be put together into a whole. However, the illumination system and the masks are primarily used for laser annealing, and cannot be used for the fabrication of LED devices.
The existing photolithographic tools for the exposure of patterned sapphire substrates are associated with a low yield, poor adaptability and unsuitability for mass production due to lens design cost, control system design and other reasons.